The present invention is in the technical field of the photoplethysmographic (PPG) measurement systems and apparatus using optical sensors. More particularly, the present invention is in the technical field of ambient noise cancellation during PPG signal measurement by optical sensors used in pulse oximeters and other devices).
Photoplethysmography is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which scatters light through a portion of the patient's tissue-where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The changing light characteristics can be measured and used to determine the heart rate of a patient and other parameters (blood oxygen saturation SpO2, respiration rate, blood pressure, etc.).
In the field of photoplethysmography, light pulses from different portions of electromagnetic spectrum are used to noninvasively determine various blood values. Typically PPG measurement systems, such as pulse oximeters, include an optical sensor for releasable attachment to the tip of patient's appendage (e.g., a finger, earlobe and others). The sensor directs light signals into the appendage where the sensor is attached. Some portion of light is absorbed and a remaining portion passes through patient tissue. The intensity of light passing through the tissue is monitored by a sensor. The intensity related signals produced by the sensor are used to compute blood parameters.
The light scattered through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light scattered through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have typically been provided with a light source that is adapted to generate light of at least two different wavelengths, and with photodetectors sensitive to both of those wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
Known non-invasive sensors include devices that are secured to a portion of the body, such as a finger, an ear or the scalp. The tissue of these body portions is perfused with blood and the tissue surface is readily accessible to the sensor.
One problem with oximeter measurements is that in addition to receiving the light that was directed at the tissue, ambient light is also detected by the photodetector. Attempts can be made to block out ambient light, but some amount of ambient light will typically be detected. One particular concern is the light at the power line frequency of fluorescent or other lights, which is 60 Hz in the United States and 50 Hz in Europe and other countries.
Since a single photodetector is typically used, the light of different wavelengths, such as red and infrared, is time multiplexed. The detected signal must be demultiplexed. The demultiplexing frequency must be high enough so that it is much larger than the pulse rate. However, choosing a demultiplexing frequency is also impacted by the ambient light interference.
The measured signals can be distorted by various ambient noises (optical and electrical), thus resulting in potential measurement errors. Device manufacturers employ various sampling and timing strategies, which usually contain an ambient light sample, during which neither of deployed LEDs is powered. This ambient light level is measured and later subtracted from the signal. The drawback of this approach is in the overload of the receiver at certain supply voltage levels, which may limit the gain of the useful signal. This usually makes the measurement invalid and obsolete. Also, the cost and area for electronics component deployment may be significantly larger.
One problem in these solutions is that the ambient light signal and useful light signal both are amplified by the first current-voltage (transimpedance) amplifier. In this case, due to limited voltage used in mobile and wearable devices the signal to noise ratio (SNR) may be dramatically decreased.
What is needed, therefore, is a device to increase the SNR in PPG measurement systems.